Conventional gas heating appliances such as furnaces, boilers, and water heaters provide the consumer with safe, economical space and water heating, while requiring little maintenance during a long lifespan. These appliances typically use single wall galvanized vent connectors and either a masonry chimney or Type B vent pipe to vent the flue gases created during operation. The American National Standards Institute (ANSI) categorizes gas appliances based on the pressure produced in a special test vent and the difference between the actual temperature and dew point temperature of the flue gas.
A category I appliance is one which has a vent expected to operate under negative static vent pressure with a minimum of condensation. A category I furnace or boiler has an Annual Fuel Utilization Efficiency (AFUE) range of 78% minimum to approximately 83%. Moisture does not condense from the flue gas in category I appliances because the actual flue gas temperature is generally more than 140.degree. F. above its dew point temperature. Traditional draft hood equipped appliances are category I appliances. However, many mid-efficiency, fan-assisted appliances are category I appliances as well. Such appliances can be made category I appliances by adjusting the flue gas temperature to be in the same range as the traditional category I appliance, and by designing the vent system to maintain a negative pressure even in the presence of the fan. Venting systems for category I appliances typically include Type B vents, lined masonry chimneys, and single wall metal vents.
Category II appliances also operate with negative vent pressure. However, the vent gas temperature is generally less than 140.degree. F. above its dew point temperature. The condensation occurring in these appliances requires the designer to use a corrosion resistant vent to exhaust the flue gases. There are few, if any, category II appliances on the market.
Category III appliances operate with a positive vent pressure and with a vent gas temperature generally at least 140.degree. F. above its dew point temperature. Their AFUE ranges from approximately 78% to 83%. Because the pressure in the vent exceeds that of the surrounding atmosphere, these appliances require an airtight vent to prevent leakage of flue gases before they reach the outside venting location. An example of a category III appliance would be a mid-efficiency furnace that is vented horizontally through the wall. Venting systems for category III appliances typically include high temperature plastic and single wall stainless steel metal vents.
Category IV appliances include furnaces, boilers, and other devices that operate with a positive vent pressure and with a vent gas temperature less than 140.degree. F. above its dew point temperature. They generally have AFUE values above 83%. Because the pressure in the vent exceeds that of the surrounding atmosphere and because condensation occurs in the vent, these appliances require an airtight, corrosion-resistant vent that is equipped for condensate disposal. Category IV appliances are usually high-efficiency, condensing devices. Venting systems for category IV appliances typically include high temperature plastic, polyvinyl chloride ("PVC"), or chlorinated polyvinyl chloride ("CPVC") vents.
ANSI Z21.47-1993 provides the current category certification requirements for gas furnaces. These requirements define and use the concept of Steady State Thermal Efficiency (SSTE) in making a category determination. SSTE measures the appliance's operating efficiency by dividing the total gas energy input to the appliance into the amount of energy gainfully used by the appliance (essentially one minus the amount of energy expelled up the flue (wasted energy)) while the appliance is operating in a steady state. AFUE, on the other hand, is an overall assessment of an appliance's annual operating efficiency. ANSI Z21.47-1993 uses flue gas temperature and the flue gas carbon dioxide content to distinguish between category I and non-category I appliances based on a SSTE of 83%. The flue gas temperature of an appliance with a given SSTE varies with the amount of excess air used for combustion and the amount of dilution air added prior to the vent. These amounts, in turn, determine the percentage of carbon dioxide in the flue (7-9% for most appliances). The ANSI specification indicates, for example, that an appliance having between 7-9% carbon dioxide in the flue gas qualifies as a category I appliance when the flue gas is approximately 140.degree. F. or more above its dew point temperature.
Assigning an appliance to a specific category is important because the category determines the type, size, material, and installation requirements of the venting system for that specific appliance. For example, a category I appliance may use traditional venting materials such as Type B vent pipe or a masonry chimney, while a category IV furnace requires a vent system built from corrosion resistant materials, and category III and IV appliances require airtight vent systems.
The flue gas of natural draft appliances, such as furnaces and water heaters, contains a large amount of water vapor. As the industry has moved to high efficiency appliances, and subsequently to lower flue gas temperatures, condensation of water and corrosive substances from the flue gas onto exhaust conduit surfaces has become a major design issue. Most new appliances are connected to an old vent, often using a single wall vent connector. In many cases, the vent is a masonry chimney. However, in today's building codes, the use of single wall metal vent connector is severely limited, and most masonry chimneys require relining before the new appliance may be installed. Converting to a Type B connector from a single wall connector may cost the building owner up to $60.00, while relining a chimney to protect against condensation can cost from around $200 to $800. For another example, problems with category III appliances using high temperature plastic vents have prompted some jurisdictions and some appliance manufacturers to prohibit the use of high temperature plastics. Alternative stainless steel vent systems cost roughly twice as much as high temperature plastic systems, in the $100 to $300 range. In short, the existing vent may be completely inadequate for the new appliance and may either prevent the building owner from installing gas appliances or require the building owner to undergo an expensive and time consuming vent system replacement.
In an attempt to avoid these costs, several manufacturers have designed appliances with draft hoods that entrain dilution air into the vent. Entraining dilution air into the vent reduces the amount of condensation formed during operation and therefore reduces the number of installations which would require chimney relining. Unfortunately, this process also allows heated room air to escape in an uncontrolled fashion, both while the appliance is operating and while the appliance is idle. The escaping heat increases the heat load on the building and therefore increases the energy cost associated with controlling the building temperature. In addition, the typical draft hood equipped appliance is susceptible to backdrafting, which is especially troublesome in the multi-story housing market.